Roof area to gutter water drainage calculations

briaineo

Hi

Just wondering if anyone is knowledgeable on roof surface water drainage, have a 13m x 7m flat roof area and I wished to slope it to the 7m section with a box gutter 150mm x 150mm.

Our engineer is saying we may have drain both ends to a center point in a butterfly type shape, this kind of ruins the effect we were looking for as it doubles the thickness of the roof with the thickness of the firring. Cold roof build up.

Is there an online calculator or a general rule of thumb regarding the roof surface area to the gutter?

Pitch of roof will be between 1 in 80 and 1 in 40

From my calculations it will work out as 1 in 60 (approx a block higher at the front)

Thanks

rayjdav

http://www.alumascrainwater.co.uk/calculator/confirmation/

This can be used as a rough calculation but cold flat roof, listen to your eng...

BryanF

Why are you second guessing your engineer?

hesker

BryanF wrote: »

Why are you second guessing your engineer?

Maybe he wants to seek a second opinion because there are consequences.

What’s the alternative. Blind subservience?

Gumbo

hesker wrote: »

Maybe he wants to seek a second opinion because there are consequences.

What’s the alternative. Blind subservience?

Lets say he gets an opinion from someone on the internet that differs from the Engineer, or a tool online that quotes a different surface run off figure.

What's he going to do?
If the Engineer wants a different set up, then that's what has to be pout in to get compliance /certification.

sydthebeat

hesker wrote: »

Maybe he wants to seek a second opinion because there are consequences.

What’s the alternative. Blind subservience?

do you value the free advice from an anonymous person from the internet as the same as a paid tangible engineers calculations?

hesker

Armed with some information you can at least have a more detailed discussion. If the engineer is insisting on it because it’s clear alternatives will not work then that's a different matter but the op did not look like that.

Calahonda52

91 sq m of flat roof may be uninsurable in any event

briaineo

Hi thanks for the replies

I’m not second guessing the engineers calculations I’m wondering what formula is used so I may come up with an alternative.

Just to say this section was always the same size but originally had planned to go with a Tata steel standing seam: but lowered the pitch and now going with a membrane type system.

The run off was the same if not more with the higher pitch on the steel roof - now that we moved to a membrane it’s not possible to flow the water this way!

The reason I was asking about the calculations is that I wanted to see if these may give an option of an alternative: so if I doubled the size of the gutter could this work! Etc.

Have insurance sorted the entire roof

BryanF

Ring membrane system manufacturer - if their reputable, they’ll do the calculation

briaineo

Actually there was a slight misinterpretation between the carpenter, myself and the engineer

Turns out the engineer wants an additional drain off to the side (middle) - carpenter understood this as the only drainage point which would have altered the rear pitch oirientation the opposite as planned

Thanks for the replies

Metric Tensor

Nice to see a follow up post - we don't often get those here!

BryanF

Assume Scupper is provided.

magnethead

I realize you got your answer, but in case someone happens to drop in here from the title of your thread

I found this site good for getting your water flow rate off the roof....about 35mm/hr is the 100 year Dublin downpour, 10 times that for Galway obviously ;p
https://www.roof-gutter-design.com.au/

20 auzzi quid to get how many downpipes at a given diameter, you'll need. or box one's